1. Field of the Invention
This invention relates to a cooling arrangement for magnetic coils. This invention further relates to a cooling arrangement for a toroidal magnetic field coil assembly. More specifically, the present invention relates to a cooling arrangement for a toroidal field coil assembly for a tokamak fusion reactor. The invention further relates to a method of cooling an assembly of magnetic coils.
2. Background of the Invention
Positioned within the plasma fusion region of a tokamak reactor is a plasma which is composed of a mixture of positively charged nuclei and free electrons. To maintain purity and to prevent instability, the plasma cannot be allowed to contact any other matter. Thus, the core of the tokamak fusion reactor (TFR) is in a hermetically sealed vacuum chamber. In addition, means must be provided to prevent the fusible nuclei from touching any structural members or walls before they have had sufficient opportunity to collide and fuse. However, at temperatures in the range where fusion reactions will occur, the nuclei are moving so rapidly that they would traverse the distance between structural members or walls in the plasma fusion region in less than a microsecond.
Thus, a non-material means must be found to contain the plasma long enough for the nuclei to collide and fuse. One approach is to employ a magnetic field to confine the hot plasma. In a tokamak device, a circular ring of plasma is generated and maintained within a toroid-shaped region by the action of intense magnetic fields which themselves are shaped to form the toroid. The magnetic field acts as a non-material container liner that insulates the hot plasma from any walls or structural components of the tokamak. The magnetic field exerts an effective pressure on the contained plasma that is proportional to the square of the magnetic field strength. By maintaining this magnetic pressure at a greater value than the internal pressure of the plasma, containment is possible.
In order to develop a magnetic field of sufficient density and intensity to contain hot plasma, large amounts of power are used to energize the magnetic coils that comprise the toroidal field coil assembly. In spite of the fact that the coils are typically made of a high conductivity material, the large currents involved create very considerable amounts of Joule heat.
In addition, in a compact TFR, such as that disclosed in U.S. Pat. Nos. 4,367,193 and 4,363,775,; where the toroidal field coils are positioned adjacent to and surrounding the plasma fusion region, they experience considerable nuclear heating from the neutrons generated in the plasma fusion region. Thus, it becomes necessary to provide a means to cool the toroidal field coil turns to prevent their destruction.
Furthermore, in a compact TFR, the toroidal field (TF) coils are of a compact size, and in order to generate the required toroidal field must carry a very high current density. Furthermore, the compact size puts additional constraints on designing effective cooling arrangements for the TF coils.
It will be apparent that flowing coolant must be distributed in the toroidal field coil assembly of the compact TFR to remove the Joule and nuclear heat generated therein. The coolant must be distributed to the various cooling channels in the TF turns in such a way that the coolant distribution through the coil turns can be controlled to enhance the cooling characteristics.
In U.S. Pat. No. 4,116,264 to Farfaletti-Casali et al, a modular toroidal assembly for forming a blanket structure for a fusion reactor is disclosed. The coolant header structure for the blanket illustrated appears to consist of several large tubes branching off of a ring-shaped manifold and entering a partial ring structure containing many small cooling tubes. The header tubes of U.S. Pat. No. 4,116,264 do not communicate with any toroidal field coils along the sides of a coil turn nor do the cooling channels bend out of the plane of the coil turns to join the header tubes thereby allowing for applicant's thermal symmetry and reduced bulk material temperature variation. In fact, no toroidal field coil structure is even disclosed in U.S. Pat. No. 4,116,264. Therefore, the cooling channels of U.S. Pat. No. 4,116,264 cannot be an integral part of any toroidal field coil as is a feature of the present invention.
In U.S. Pat. No. 4,268,353 to Powell et al, there is disclosed a superconducting toroidal field coil assembly which contains cooling channels. However, those coils being both superconducting and located outside the region of the blanket and shield means are not subject to nuclear heating nor to intense Joule heat as are Applicants' TF coils. Moreover, the enormous size of those coils in comparison to the compact coils of the present invention renders the space required for cooling passages within them less crucial as a design consideration. Because of these looser design constraints, the coolant inlet passages of Powell et al are permitted to traverse the TF coil in a direction parallel to the flat face of the coil. Such a design would be unacceptable in a compact device since it would entail the removal of relatively large amounts of coil material in a local region. This in turn would result in hot spots and high stresses.
Burgeson et al in U.S. Pat. No. 4,277,768 disclose coolant means for their TF coil, each channel of which completely traverses the TF coil from the inner contour to the outer contour thus requiring the creation of large voids for coolant flow in the TF coil which, as explained above, would be unacceptable in Applicants' compact device. Such a design is possible in the device of Burgeson et al because of the massive size of the coils, the location of the coils remote from the plasma region, and the use of superconducting TF coils.